A multiple-screw extruder is a commonly used instrument for the plastication, mixing and/or pumping of extrudable material, such as polymeric material or other kinds of viscous material such as dough. Polymeric materials are useful for the fabrication of a variety of molded or shaped articles. Plastication refers to the softening of polymeric material to such an extent that it flows freely and will assume any shape. In the case of polymeric material which is crystalline, plastication is synonymous with melting. In the case of polymeric material which is amorphous, plastication occurs at or about the glass transition temperature ("T.sub.g ") thereof. (Reference below to "polymeric material" may be read also as a reference in the appropriate sense to other kinds of extrudable material.)
A multiple-screw extruder contains two or more screws, each of which is similar in various basic respects to that which is used in a single-screw extruder. Such a screw is shaped generally in the form of an elongated cylinder, and has one or more raised ridges helically disposed thereabout, each of which is a "flight". A flight may have forward or reverse pitch, and the degree of pitch may be varied. The surface of the screw above which the flight(s) are raised is the root of the screw. When the screw is viewed in cross section (i.e. in a plane perpendicular to its central, longitudinal axis), the course of a particular flight, between one point of intersection with a line parallel to the screw axis and the next closest point of intersection of the flight with such line, typically defines a 360.degree. circle. The pointed or rounded tip of a flight which extends toward the perimeter of such circular-shaped cross section defines a "lobe" above the root of the screw. In the cross-sectional view of FIG. 1, the lobes 10 of a 2-flighted screw are shown. The distance in the longitudinal (axial) direction along the screw, between one point on a line intersecting a particular flight and the next closest point of intersection of such line with the same flight, is one turn of the screw. The space bounded by the root of the screw and the side walls of any two flights is a channel of the screw. The side wall of a flight may, if desired, be undercut such that the channel does not, as so modified, conform to the Erdmenger profile. The screw rotates on its longitudinal axis within a barrel or sleeve, which may be generally described as the bore of an annular cylinder.
The plasticating screw typically has an initial, feed section which begins the process of conveying solid polymeric material forward within the barrel of the extruder. Polymeric material may be flood fed into the extruder by means of a hopper which empties into the barrel, or may be metered into the barrel through a feed chute or a side feeder. The direction of travel of the polymeric material in the barrel as it is transported away from the feed port by the screw is considered the "downstream" direction. The feed section of the screw is typically followed, with or without other intervening sections, by a melting section in which partial or complete plastication of the polymeric material occurs. Plastication of the polymeric material occurs as a result of the combined effect of conductive heat produced, for example, by heater bands mounted on the outside of the extruder barrel and the physical deformation (or viscous heating) to which screw action subjects the polymeric material. The melting section of the screw is typically followed, again with or without other intervening sections, by a metering section which functions to pump the plasticated material, as extrudate, out through the downstream end of the extruder, which typically contains a die or some other form of restricted orifice. The sections of the extruder and screw through which the polymeric material travels before it reaches the die are considered to be "upstream" from the die.
Aspects of a multiple-screw extruder which have no direct analogy in a single-screw extruder pertain to the location, relative to each other, of the several screws within the barrel. Two screws in a multiple-screw extruder are said to be intermeshing if a flight of one screw is disposed within a channel of the other screw. In such configuration, the distance between the axis of each screw is less than the sum of the respective radii of the two screws, when each radius is measured from the axis to the top of the tallest or highest flight of the screw. A pair of conventional intermeshing screws is shown in FIG. 2. When, on a pair of screws, a flight has a shape and size such that its fit into a channel in which it is intermeshed is close enough that essentially no extrudable material passes through the space between the flight and channel, the screws are said to be conjugated. Otherwise, the screws are said to be non-conjugated, and the degree of intermeshing in the case of non-conjugation can be varied to an essentially unlimited extent. If the screws rotate in the same direction, i.e. both clockwise or both counterclockwise, they are said to be corotating. If not, they are said to be counterrotating.
When two counter-rotating screws are conjugated, there is little or no opportunity for polymeric material which is being extruded to remain in a channel of either screw in the region in which the screws intermesh. This is a result of the minimal clearance of the flight of one intermeshing screw within the channel of the other screw. Screws which are arrayed in such manner are said to "wipe" each other because polymeric material which is carried in the channel of either screw will be prevented from remaining in that channel over a distance of more than one turn of the screw by the obstacle which the intermeshing flight of the other screw represents. In this configuration, most, if not essentially all, of the polymeric material resides in the portion of the channel on the top, bottom and outside (non-intermeshed side) of each screw, and the material consequently experiences significantly more movement in the downstream direction, parallel to the screw axis, than it does in a lateral direction, perpendicular to the axis. This arrangement results in efficient pumping of the polymeric material in the downstream direction, but does not achieve optimum intermixing of the whole body of polymeric material present in the extruder. Conversely, counter-rotating screws which are non-conjugated are characterized by greater clearance between an intermeshed flight and the corresponding channel. This allows greater space for polymeric material to reside in the channels of both screws in the region of intermeshing. Intermixing of the polymeric materials is enhanced by this concentration of the action of both flights in such a confined space, however some efficiency of pumping is lost.
Corotating screws, even when conjugated, allow for extensive movement of polymeric material laterally from one screw to the other. Mixing is benefited by this movement around the "equivalent screw", and it is further enhanced when the screws are not conjugated. The shape of the flights on non-conjugated screws may be arranged to create the passage of polymeric material from one channel into two channels on another screw. Or, when screws are conjugated, or essentially conjugated, certain flights can be designed in a shape such that they wipe each other in the zone of intermeshing but do not wipe the wall of the barrel. For example, a multiple-screw, barrier-type extruder is disclosed in U.S. Pat. No. 4,300,839 in which one or more flights on each screw is reduced in height by an amount which is constant along the entire length of the screw, and which is sufficient to allow the passage of polymeric material between the two channels on either side of any such flight. This clearance between the wall and the top of the flight does not exist, however, for those flights in the zone of intermeshing. Designs such as these appear to provide thorough mixing, but considerations of pumping efficiency and residence time sometimes limit their usefulness.
Nichols et al disclose in Modem Plastics, Volume 61 at Page 70 (February, 1984), that dissipative mixing can be attained in a counter-rotating tangential twin-screw extruder by staggering the screw flights. Unfortunately, this technique appears to have no application beyond the field of tangential (i.e. non-intermeshing) extruders because the flights of conventional intermeshing screws may be considered, in this context, to be "staggered", at least to some extent, in the sense that they could not intermesh if a flight on one screw was not paired with a channel on the other screw rather than another flight.
It would therefore be beneficial to have a design for two intermeshing screws to be paired in a multiple-screw extruder to simultaneously obtain both a desirably high degree of intermixing, and a high rate of pumping, of extrudable material. It is consequently an object of this invention to provide a multiple-screw extruder in which even conjugated screws will furnish a desirably high level of compositional homogeneity and process stability.